AlxGa1−xN/GaN heterostructures are of great interest for use in semiconductor devices such as, but not limited to, high electron mobility transistors, HEMTs (also known as heterostructure or heterojunction field effect transistors, HFETs) due to excellent high frequency and power handling capabilities.
The transport properties of AlxGa1−xN/GaN heterostructures have been studied intensively over the last two decades. At high temperature regime (>100 K), which is of interest for most device applications, the mobility of the two-dimensional electron gas (2DEG) formed near the interface of the AlxGa1−xN/GaN heterostructure has been theoretically calculated and has shown to be ultimately limited by phonon scattering (L. Hsu and W. Walukiewicz, Physical Review B 56, 1520 (1997), L. Hsu and W. Walukiewicz, Journal of Applied Physics 89, 1783 (2001)).
Nevertheless, in practice, other important scattering mechanisms related to structural imperfections of the materials, including alloy disordering and interface roughness, can actually dominate the 2DEG mobility.
Room temperature 2DEG mobilities of 1300 to 1600 cm2/Vs have been reported for AlxGa1-xN/GaN heterostructures, depending on the 2DEG density and crystalline quality (N. Maeda et al, Optical Materials 23, 211, (2003), and V. M. Polyakov et al, Applied Physics Letters 97, 142112 (2010).
By insertion of a thin (˜1-2 nm) aluminum nitride exclusion (AlNex) layer between the GaN and the AlxGa1−xN layers, the 2DEG mobility can be increased to ˜2200 cm2/Vs (R. S. Balmer et al, Phys. Stat. Sol. 7, 2331 (2003), X. Wang et al, Journal of Crystal Growth 298, 835 (2007) and U. Forsberg et al, Journal of Crystal Growth 311, 3007 (2009)). This improvement is associated with better 2DEG confinement and less alloy disorder near the AlxGa1−xN/GaN interface, enabling less electron wave function penetration into the AlGaN barrier so that alloy disorder scattering is alleviated.
A drawback of insertion of an AlNex exclusion layer is that it may, due to its wide band gap nature, increase the surface potential in a HEMT structure, resulting in that a low ohmic contact is difficult to obtain. The low ohmic contact is essential for high frequency applications. Additional recess etching into the AlxGa1−xN barrier for contact metallization process becomes necessary to reduce the contact resistance below 0.5 Ωmm.
Hence, it is highly desirable to develop an AlxGa1−xN/GaN heterostructure with high mobility, but without disadvantages of prior art AlxGa1−xN/GaN heterostructures, and a method for producing the same.
Moreover, it is highly desirable to develop an AlxGa1−xN/GaN heterostructure for semiconductor devices resulting in devices with higher operation frequencies as well as with less trapping and lagging effects.
Example of a prior art method is disclosed in US 2015/069407 A1.